Certain elements of the prior art or of the invention are described here in a spatial frame of reference related to the aircraft, referred to as aircraft frame of reference. Throughout the description, this aircraft frame of reference is defined in the usual way by a longitudinal direction of the aircraft, a transversal direction of the aircraft and a third direction, orthogonal to the other two, which by convention is referred to as vertical direction, even though it does not coincide—at least during flight—with the “vertical” of a terrestrial frame of reference such as defined by gravity. When any doubt is possible as to the frame of reference in question, the “vertical” of the terrestrial frame of reference is referred to as gravity direction.
Furthermore, the term “wind” designates the total air movement at a given point, which results from superposition of the mean air movement (laminar flow) and of the turbulence at that point. Turbulence is an agitation composed of complex and disordered movements, constantly changing.
Turbulence has detrimental effects on the aircraft. In particular, it may induce: vertical accelerations of the aircraft, capable of displacing objects or passengers in the cabin; a change of altitude levels, which in particular may cause a risk of collision with another aircraft; excess loads on the wing group; large roll moments; a sensation of discomfort in the cabin, etc.
Three types of turbulence in particular are responsible for problems caused for the aircraft:                clear air turbulence, which results from wind shear; this turbulence, non-convective, appears at high altitude close to the jet streams, most often above mountains and more likely in winter,        convective turbulence, which appears inside or close to clouds; very severe turbulences may occur in storm clouds, where there coexist vertical currents in opposite direction that may reach tens of m/s. These phenomena are local and in general are visible (because of the presence of the clouds).        wake turbulence, created by the passage of an aircraft; the vortices generated by a heavy aircraft may induce large roll moments on a lighter aircraft.        
Because they increase the loads on the wing group, turbulences make it necessary to reinforce the aircraft structure; consequently they have an impact on the weight of the aircraft. In addition, turbulences fatigue the aircraft structure and, because of this fact, limit its useful life or at the very least detract from its operational profitability by necessitating frequent inspections of the structure and equipment items of the aircraft. Also, and above all, turbulences are the primary cause of injuries among passengers, not including fatal accidents.
The detection and measurement of turbulences as well as the employment of corresponding remedial actions therefore represent high stakes.
It is known that the effects of turbulence on an aircraft can be attenuated by actuating mobile control surfaces of the aircraft, such as ailerons, flaps, spoilers, slats, elevators, rudders, elevons, etc., so as to limit the load variations to which the aircraft is subjected.
FR 2891802 additionally teaches that the effects of vertical turbulence can be attenuated by calculating a control instruction for a mobile control surface according, on the one hand, to the vertical component of wind speed at the current position of the aircraft and, on the other hand, to a level of severity of the vertical turbulence occurring at the current position of the aircraft, this level of severity being calculated on the basis of the aforesaid vertical component of the wind. In practice, when the aircraft is an airplane, the vertical component of the wind is measured by an anemometric sensor placed on the nose of the airplane, in order to know with a short lead time the wind to which the airplane wings will be subjected. Nevertheless, even in the largest airplanes, this lead time remains shorter than 100 ms. Considering the output speed of the known actuators, it is therefore not possible to turn control surfaces situated on the wings to full deflection.
To remedy this disadvantage, it is also known that lidars (acronym for “Light Detection and Ranging”, meaning detection by light waves and telemetry) can be used to measure wind speeds ahead of the aircraft at a given distance therefrom, with a view to detecting the turbulences occurring at that distance. A lidar is an active transducer comprising a laser that emits a directed incident light beam, a telescope that collects the wave backscattered by the particles encountered by the incident beam, and processing means. The backscattered wave collected at the instant t=2d/c (where “c” denotes the speed of light) after emission of an incident beam corresponds to the wave backscattered by the atmospheric layer situated at the distance “d” from the lidar, referred to as sight distance. According to the Doppler effect, the speed of displacement of the said atmospheric layer in the sight direction of the lidar is deduced from the difference between the frequency of the incident beam and that of the backscattered wave. The measurement of the wind at a given distance ahead of the aircraft makes it possible to evaluate the phenomena that will affect the aircraft with a lead time longer than 100 ms, and therefore offers the possibility of turning a control surface to full deflection if necessary.
Nevertheless, the known devices and methods employed for attenuating the effects of turbulence are not entirely satisfactory.